Surface Properties of Al-Functionalized Mesoporous MCM-41 and the

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The surface properties of Al-functionalized mesoporous MCM-41; the melting behavior of water in Al-MCM-41 nanopores Angelina Sterczynska, Anna Derylo-Marczewska, Malgorzata ZienkiewiczStrzalka, Malgorzata Sliwinska-Bartkowiak, and Kamila Domin Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02172 • Publication Date (Web): 29 Aug 2017 Downloaded from http://pubs.acs.org on September 4, 2017

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The surface properties of Al-functionalized mesoporous MCM-41; the melting behavior of water in Al-MCM-41 nanopores Angelina Sterczyńska

a,b

, Anna Deryło-Marczewska c, Małgorzata Zienkiewicz-Strzałka c, Małgorzata Śliwińska-

Bartkowiak a,b*, Kamila Domin a,b a

Faculty of Physics, Adam Mickiewicz University, Umultowska 85, 61-614 Poznan, Poland;

b

The NanoBioMedical Centre, Umultowska 85, 61-614 Poznan, Poland;

c

Faculty of Chemistry, M. Curie-Skłodowska University, M. Curie-Skłodowska Pl. 3, 20-031 Lublin, Poland

Corresponding Author * E-mail: [email protected], Phone: +48 618295235

KEYWORDS

Silica mesopores, Al-functionalization, potentiometric titration, XPS spectroscopy, contact angle, melting in pores

ABSTRACT We report an experimental investigation of structural and adhesive properties for Al-containing mesoporous MCM-41 and MCM-41 surfaces. In this work, high-ordered hexagonal mesoporous structures of aluminosilica with two different Si/Al molar ratios equal 50 and 80 and silica samples were studied; Al was incorporated in the MCM-41 structures using the direct synthesis method, with CTAB as surfactant. The incorporation of aluminium was evidenced simultaneously without any change of the hexagonal arrangement of cylindrical mesopores. The porous materials were examined by the techniques, such as: low-temperature nitrogen sorption, energy dispersive spectroscopy, and also scanning and transmission electron microscopy. Surface

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properties were determined through X-ray photoelectron spectroscopy, potentiometric titration and static contact angle measurements, respectively. It was shown, that an increase of surface acidity leads to the increase of the wetting energy of the surface. To investigate an influence of acidity on the confinement effects, the melting behavior of water in Al-MCM-41 and MCM-41 with the same pore size was determined by using dielectric relaxation spectroscopy and differential scanning calorimetry methods. We found that the melting point depression of water in pores is larger in the functionalized pores relative to pure silica pores of the same pore diameter.

INTRODUCTION Increasing interest in mesoporous silica materials with the highly ordered pore structure is observed in recent past two decades. In 1998, Zhao et al. reported the synthesis a new type of SBA-15 material characterized by twodimensional hexagonal structure [1] with the thicker pore wall relatively to mesoporous MCM-41. Intensive studies of these materials are related with their unique properties such as perfect channel system, large surface area, uniform pore size, thermal and chemical stability [2-4] and also the possibilities of their extensive applications as sorbents for separation problems, drugs delivery problems or in sensor design and as optical devices [5–10]. Mesoporous matrices have found broad range of applications as catalysts in reaction of molecules because of their relatively high surface area (~ 1000 m2/g) and wide pore sizes [11]. The majority of these materials are obtained in hydrothermal processes with the organic templates [12–14]. The purely siliceous mesopores contain only silanol groups with the low acidity and are rather catalytically non-active. The substitution of heteroatoms into siliceous framework and modification of the silica walls is recently widely investigated [15–20]. The addition of heteroatoms such as aluminium (with lower than silicon valence) introducts the negative charges in the silica walls what is compensated by protons, and generates an acidity in these systems, improving surface properties of the final product [21]. An introduction of an aluminium to the reaction gel by direct synthesis methods leads to an appearance of tetrahedral and octahedral forms of aluminium [22]. As follows from many investigations, Al-SBA-15 kind of materials show much higher catalytic properties than Al-MCM-41 [23–25]. Nowadays, it is still a challenge to find the best synthesis procedure to obtain porous systems with appropriate high content of Al to enhance their acidity. In 2004, Vinu et al. described a new technique of the synthesis of high Al content SBA-15 porous matrix, where by an adjustment of an appropriate nH2O/nHCl molar ratio, the decreasing of the synthesis solution pH was observed [26].


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We report an investigation of mesoporous silica matrices before and after functionalization with aluminium. The surface properties of MCM-41 and Al-MCM-41 obtained using hexadecyltrimethylammonium (CTA+) species as the structure-directing surfactant agents are characterized. Morphological and nano-structural characterizations of MCM-41 and Al-MCM-41 were performed by low-temperature nitrogen adsorption (77 K), transmission electron microscopy (TEM), elemental analysis (EDS) and also by scanning electron microscopy (SEM). In order to verify the surface charge of the samples, we used the potentiometric titration method. The surface properties were also investigated by X-ray photoelectron spectroscopy and contact angles analysis. An influence of the acidity on the melting phenomenon of water confined in Al-MCM-41 was studied based on the differential scanning calorimetry (DSC) and dielectric relaxation spectroscopy (DRS) techniques.

1.

EXPERIMENTAL SECTION

1.1 Materials. Hexadecyltrimethylammonium bromide (CTAB, 99%), tetraethyl orthosilicate (TEOS 98%) and 1,3,5-trimethylbenzene (TMB) were purchased from Sigma-Aldrich; sodium hydroxide (NaOH) and aluminium chloride (AlCl3) were obtained from Polish Chemical Reagents. In all experiments bi-distilled water was used.

1.2 Preparation of Al-MCM-41 by direct synthesis procedure. A sample of Al-MCM-41 was obtained by modified synthesis described by M. Hornacek et al. [27]. We have replaced the aluminium sulfate by an aluminium chloride as Al-source. We applied the following gel composition: n(CTAB) : n(Si) : n(H2O) : n(Al) = 0.2 : 1: 120 : 0.01 and the Si to Al mole ratio in this gel was equal 50:1.

1.3 Synthesis of MCM-41. This kind of silica matrix was obtained according to the method recommended by Trofymluk et al. [28]. In order to eliminate surfactant molecules from Al-MCM-41 and MCM-41 products, the samples were filtered off, washed with bi-distilled water, then they were dried and calcined in air at 873 K for six hours.


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1.4 Methods of characterization

1.4.1 Nitrogen Adsorption Experiments. Nitrogen adsorption/desorption isotherms at 77 K were studied using an ASAP 2020 analyzer (Micromeritics Corp. Norcross, GA, USA). Prior to the measurements, the adsorbents were degassed (0.01 mm Hg) at 423 K. The pore sizes and the porosity size distribution (PSD) of samples of: MCM-41 and Al-MCM-41 materials were calculated using two methods: BJH [29] as well as NLDFT method. The results obtained by Non-Local Density Functional Theory (NLDFT) were calculated accordingly to the model for cylindrical pores in an oxide surface, using non-negative regularization of 0.0316. The diameters of mesopores were evaluated from the PSD maxima from adsorption and desorption branches. The BET specific surface area, SBET, was calculated by using the linear BET plots of adsorption data. The total pore volume, Vt, was obtained at the ratio p/p0=0.98 [30]. The values of the external (macroporous) surface area, Sext, and primary pore volume,Vp were estimated from the αs-plot method, where as a reference non-porous adsorbent the macroporous silica gel LiChrospher Si-1000 was applied [31].

1.4.2 Energy dispersive X-ray spectroscopy. The chemical composition of aluminosilicate sample has been determined from the SEM-EDS method at voltage 15 kV. An optimization element was the silicon.

1.4.3 Electron microscopy. TEM and SEM micrographs of MCM-41 and Al-MCM-41 samples were harvested with a JEOL JEM-1400 and JEOL JSM-7001F instruments, respectively, which typical specifications and sample preparation were described elsewhere [36]. TEM images of large pores MCM material (MCM-41-TMB) were collected by Tecnai G2 T20 X-TWIN (FEI) microscope.

1.4.4 Photoelectron spectroscopy. The surface composition as well as interaction between general elements in Al-MCM-41 structure has been characterized by X-ray Photoelectron Spectroscopy (XPS). XPS is a method for surface investigation of materials and provides information about their chemistry (chemical state in terms of quantitative effects). XPS allows to define the binding energy of the atoms in the sample and describe the dependences between the shared electron pairs of individual atoms. This technique is able to identify any changes in binding energy values of electrons on core shells after any chemical changes in atoms background. As a consequence on XPS spectra are visible peaks with shifts of energy; this chemical shift defines types of chemical bindings between the atoms of the sample.


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1.4.5 Potentiometric Titration. Potentiometric titration experiment was conducted with a 665 Dosimat autoburette (Metrohm, Herisau, Switzerland) using a PHM 240pH-Meter (Radiometer, Copenhagen, Denmark). The known amount of material (about 0.1 g) was mixed with 30 cm3 of NaCl of ionic strength 0.1 mol/dm3 in a gas-tight vessel thermostated at 293±0.1 K and equilibrated for several hours. After each titration step, the drift of the pH value was measured continuously, and the next step was performed only if the drift was smaller than 0.001 – 0.0001 pH/min. The titration curves were transformed into the surface charge density curves based on the equation: Qs =

F∆n S BET

(1)

In the above Qs is the surface charge density, ∆n is the change of H+/OH- balance per mass of a solid, F is the Faraday number. 1.4.6 Contact Angle Measurements. We have determined the wettability of aluminosilica relatively to pure silica materials, compressed in a press under a pressure ca. 700 MPa. Wettability for eight divergent liquids including water, heavy water, octamethylcyclotetrasiloxane (OMCTS), carbon tetrachloride, n-hexane, polyethylene glycol (PEG 10 000), mercury and gallium deposited on these tablets was measured by a Phoenix 300 Contact Angle Analyzer (SEO) using the sessile drop method. The static contact angles were obtained using tangent line method [32]. Measurements were conducted at ambient temperature (298 K).

1.5 Differential Scanning Calorimetry (DSC) and Dielectric relaxation spectroscopy (DRS) investigation We have performed experimental studies of the melting behaviour of water confined in Al-MCM-41 matrix, to compare the melting temperature of the liquid in pores of aluminosilanoceous walls with the melting temperature in the pure silica pores. Dielectric relaxation spectroscopy and differential scanning calorimetry methods in wide frequency and temperature range were performed. Before an experiment, in order to remove the air prior to and during an adsorption of the studied water, the porous samples were heated to about 400 K and stored under vacuum (10-4 Tr) for a few days. 1.5.1 Differential Scanning Calorimetry. In the aim to determine the melting temperature of the water confined in Al-MCM-41, a DSC 8000 Perkin Elmer differential scanning calorimeter was applied. The DSC scans were obtained at heating rates of 5 K min-1 in temperature from about 180 K to 320 K. The temperatures of phase


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transitions were determined from the positions of the first derivative of the endothermic peaks on the heat flow signals during the heating with the reproducibility equal to 0.008 K. 1.5.2 Dielectric relaxation spectroscopy. The electric permittivity is a good indicator of an appearance of liquid phase in dipolar systems, because of its significant change in the orientational polarizability between the solid and the liquid state at the melting point [33-34]. The complex electric permittivity ε* = ε′+ iε″, where ε′ =C/C0 is the real, and ε″ = tan(δ) . ε′ is the complex part of the permittivity, was obtained using a Solartron 1260 impedance gain analyzer in the frequency range 100 Hz – 1 MHz and in temperatures from 200 K to about 300 K during the heating process. The details of this experimental setup are described in [36]. The Debye dispersion relation, describing the rotation of an isolated dipole under an oscillating field, was used to evaluate the dielectric relaxation time according to the formula [35-36]:

ε * = ε ∞` +

ε s` + ε ` 1 + ifτ

(2)

where f is the frequency of the potential applied, τ is the orientational relaxation time of a dipolar molecule and the subscripts: s and ∞ refer to the static (in the low frequency) and the optical (induced) permittivity, respectively.

2. RESULTS AND DISCUSSION 2.1 Porosity Characteristics from Nitrogen Sorption Measurement.

For all systems studied N2 adsorption/desorption isotherms are presented in Figure 1A. The data obtained from sorption experiment are presented in Table 1. As follows from Fig. 1A, they present type IV of IUPAC classification isotherms with H1 hysteresis loop. For Al-MCM-41 step in the adsorption isotherm is shifted to higher relative pressures and corresponds to higher pore diameters in relation to MCM-41 material; an average pore size is ca. 14 nm. Similar pore sizes values of studied samples were obtained by using NLDFT method. Differential pore size distribution curves obtained by applying NLDFT for Al-MCM-41 and MCM-41 samples are presented in Fig. 1 (C) and (D), respectively. The results calculated from applied BJH method well agree with pore size values obtained from NLDFT technique. Moreover, the aluminosiliceous sample is characterized by wider pore size distributions


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compared to pure silica MCM-41 (Figure 1B). As follows from Table 1, for Al-MCM-41 we observe a 71% decrease of SBET ; in the Al-SBA-15 matrices, which we studied before [36] – an introduction of Al results in the increase of SBET. It might be caused of the existence of the non-framework aluminium-ion which is deeply constituted within the silica porous wall and through it the pore size leads to increase. Such an effect was observed in the zeolites [37]. No microporosity was detected in both materials studied. As it is shown in Table 1, we can also find a distinct increase of pore volume Vp and Vt for Al-containing silica in comparison to pure one (such effect is also observed for materials where an aluminium is incorporated by direct synthesis (Kumaran 2008), [38]).

A

Halsey: Faas Correction

7

Al-MCM-41 (ads) Al-MCM-41 (des)

800

MCM-41

(ads)

MCM-41

(des)

Al-MCM-41 (ads) Al-MCM-41 (des)

6

MCM-41

(ads)

MCM-41

(des)

3

900

dV/dlogD Pore Volume [cm /g Α]

3

BJH Adsorption and Desorption dV/dlogD Pore Volume

B

Isotherm Linear Plot 1000

Quantity Adsorbed (cm /g STP)

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700 600 500 400 300 200

5 4

Adsorption 3 2

Desorption

1

100

0

0 0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

0

50

100

Relative Pressure (p/p0)

150

200

250

300

Pore Diameter [A]

D

C


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Figure 1 Al-MCM-41 aluminosilica and MCM-41 silica porosity characteristics: (A) Nitrogen adsorption/desorption isotherms at 77 K. (B) Differential distribution functions of mesopore volumes evaluated from sorption isotherms. Pore size distribution functions determined by NLDFT method in: (C) Al-MCM-41 material and (D) MCM-41 material.

Table 1 Influence of aluminium on the structure characteristics of mesoporous MCM-41 silica.

Sample

Pore Size: BJH Adsorption average pore diameter

Pore Size: BJH Desorption average pore diameter

BET Surface Area

αs-plot External surface area

αs-plot Primary pore volume

Total pore volume

[m2/g]

[cm3/g]

[cm3/g]

[m2/g] [nm]

[nm]

Al-MCM-41

16.07

12.18

594

9

1.54

1.58

MCM-41

2.65

2.56

1014

20

0.56

0.59

2.2 Morphology and structure of synthesized molecular sieves.

Transmission electron micrograph of MCM-41 shown in Figure 2B reveals the one-dimensional (1D) channels indicating hexagonally ordered pores arranged in a linear array. The widths of the channels and the distances in between them were analyzed using ImageJ program and are equal: 2.7 nm and 3.2 nm, respectively. TEM image of aluminosilicate MCM-41 (Figure 2D) confirms that rows of one-dimensional pores can still be observed. The average widths of the pores are equal to 13.2 nm and the distance between the centers of the adjacent channels is ~ 15 nm. As follows from SEM image of Al-MCM-41 (Figure 2C), functionalization has significantly modified the particle shape; moreover, the particles size of the aluminosilica (3.2 µm) is slightly higher than for pure MCM-41 silica (2.6 µm). It can be the result of lower external surface area and higher mesopore volume of Al-MCM-41 relatively to MCM-41 material.


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B

D

Figure 2 (A) SEM image of MCM-41 molecular sieves, (B) TEM image of MCM-41 matrix in longitudinal projection (along the mesopores), (C) SEM image of Al-MCM-41 molecular sieves, (D) TEM image of Al-MCM-41 matrix in longitudinal view.

2.3 Chemical composition and functional groups determination. By studies of EDS spectroscopy for Al-MCM41 samples, the chemical composition, and the content of aluminium introduced in the material were found. The data collected from the analysis of this sample are presented in Table 2. In Figure 3 the EDS spectrum is depicted for Al-MCM-41 as a dependence of x-ray counts from energy; as follows from energy peaks positions, in the sample of Al-MCM-41 are present such elements as: silicon, oxygen, carbon, nitrogen, bromine and aluminium. These peaks are narrow and well resolved. As determined from an elemental analysis, the atomic Si/Al ratio in the AlMCM-41 is 79 of atomic percent, what is in good agreement with the set molar ratio of the synthesis gel (50:1). Thereby, the rates at which the aluminium and silicon are introduced into the silica skeleton are similar, and as a result the final Al-MCM-41 product retains the Si/Al ratio of the initial gel. The congenial results were also obtained


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by Janicke (1994), where MCM-41 samples were synthesized under hydrothermal conditions with higher Si/Al ratios in the range from 16 to 64 [15]. Table 2 Quantitative results of EDS analysis for Al-MCM-41 molecular sieve.

Element

Concentration

Al

1.62

130.09 Si n(Si)/n(Al)=79.34 (Atomic %) n(Si)/n(Al)=82.95 (Weight %)

Weight %

Atomic %

1.42

0.56

117.79

44.43

Figure 3 EDS spectrum of Al-MCM-41 molecular sieve.

As follows from the XPS data for Al-MCM-41 sample shown in Figure 4 and table 3, the major peaks include C1s, O1s, Si2p, Al2p signals in position 284.5, 532.5, 103.5 and 74.5 eV, alternately. From spectra for Si photoelectrons from 2p energetic level it can be observed only one type of XPS signal without shift of binding energy. For Si2p, the binding energy 103.5 eV suggests the connections with oxygen by creating Si-O linkages in SiO4-4 units. Similarly, the binding energy of O1s (533 eV) suggests that atoms are connected with silicon in Si-O linkage. Taking into account the aluminum connections, the high resolution spectra of X-ray photoelectron confirm the presence of pair of peaks. The 2pAlA and 2pAlB peaks at 74 eV and 75.5 eV suggest the existence of different chemical state of this atoms in the Al2O3 and Al(OH)3 forms – table 4. Table 3 Quantitative results of XPS analysis for Al-MCM-41

Sample

Al-MCM-41

Name

Position

FWHM

Raw Area

Area/RSF

%Atomic % Mass Concentration Concentration

C 1s

284,5

2,43

10482,3

10482,3

2,6

1,5

O 1s

532,5

2,46

670482

228833

57

43,9

Si 2p

103,5

2,55

130026

159151

39,2

52,6

Al 2p

74,5

1,72

1562,7

2910,06

1,2

1,9


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Table 4 Detailed analysis of XPS results for Al-MCM-41. The functional groups determination

Region

Name

C 1s

Position

FWHM

% Atomic Concentration

Area

C 1s A

284,5

1,6

757,3

66,8 C-C

C 1s B

286

1,55

270,4

23,9 C-O

C 1s C

287,5

1,55

105,7

9,3 C=O

533

1,92

64124,9

100 SiO2

103,5

1,94

11559,5

100 SiO2

74

1,77

71,9

75,5

2

134,7

O 1s

O 1s

Si 2p

Si 2p Al 2p A Al 2p B

Al 2p

34,8 Al2O3 65,2 Al(OH)3

A

B

1 x 10

x 10

3 O 1s

C 1s A

190

30

185 25

180

175 20 CPS

170 CPS

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165

15

C 1s B 160 10

155 C 1s C 150

5

145 0

294

291

288 285 Binding Energy (eV)

282

538

536

534 532 Binding Energy (eV)

530

528


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C

D

2 60 x 10

34

Si 2p

x 10

1

50 32

Al 2p B

40

CPS

CPS

30

30

Al 2p A 28

20 26

10 24

107

106

105 104 103 102 Binding Energy (eV)

101

100

79

78

77

76 75 74 73 Binding Energy (eV)

72

71

E 4 30 x 10

O 1s

2 50 x 10 45

25

40 Al 2p

35

20

CPS

30 25 20 15 10 15

5 0 120

110

100

90 Binding Energy (eV)

80

70

60

1200

900

600

300

O 2s

C 1s

5

Al 2p

Si 2s

10

Si 2p

O KLL

CPS

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Figure 4 XPS spectra of elements present in the sample of Al-MCM-41: (A) C 1s, (B) O 1s, (C) Si 2p, (D) Al 2 p region and (E) whole-range XPS spectrum. An inset shows Al 2p region in magnification.

2.4 Surface Acidity. The acid-base properties of the synthesized Al-containing and pure silicas were studied on the basis of potentiometric titration, and as it was shown earlier, by XPS method. The experiment was conducted for AlMCM-41 and MCM-41, and also for Al-SBA-15 and SBA-15 [36] where the diameters of silica mesopores were 4.6 nm and 4.9 nm, respectively. For Al-SBA-15, the atomic Si/Al ratio was equal 517 [36], what is a six times lower than for Al-MCM-41 studied. In Fig. 5 the dependences of surface charge density on pH are shown for all studied materials: Al-MCM-41 and MCM-41 (Fig. 5A) and Al-SBA-15 and SBA-15 (Fig. 5B). Strong similarity of surface charge over a wide pH range is observed for Al-SBA-15 and SBA-15 samples; only a small shift of the pHPZC point (point of zero charge) towards lower pH values evidences existence of some acid centers on the surface of Al doped silica (Fig. 5B). Therefore, the introduced acid sites have an rather weak influence on the adsorptive properties of the synthesized Al-SBA-15 material, as it was shown in our previous paper [36]. In general, incorporation of heteroatoms, such as Al, introduces a charge imbalance in the silica framework which is then balanced by protons; it generates formation of bridging hydroxyl groups (Si-OH-Al unsaturated bond - Brønsted acid sites) in these materials. The negative charged sites will increase the electrostatic interactions (i.e. van der Waals, ionic and donor-acceptor interactions) between the guest and the host molecules on the surface, improving the adsorption and catalytic properties of silica matrix. As follows from Fig. 5A, in the case of MCM-41 type materials the obtained results are quite different, the surface charge curves corresponding to both materials are different than in Fig.5B. This result is in good agreement with the XPS data of Al-MCM-41, which have shown the presence of Al(OH)3 and Al2O3 groups. It was shown [39] that Al(OH)3 groups are mainly responsible for increase of hydrophilic interactions on the surface. The presented Qs(pH) dependences show that Al-MCM-41 is more acidic than Al-SBA-15 because of the sample contains relatively high number of aluminium acidic sites. We observe that for Al-MCM-41 its pHPZC point is shifted about 1.5 towards low pH values relatively to Al-SBA-15. This shift of surface charge evidences large differences of surface properties of


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both studied materials. This conclusion is in accordance with EDS data confirming higher Al content in Al-MCM-41 silica in comparison to Al-SBA-15 [36].

A

B 0,1

0,05

0,05

0

0

Qs [C/m2]

0,1

Qs [C/m2]

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-0,05

-0,05

-0,1

-0,1 MCM-41

-0,15

SBA-15

-0,15

Al-MCM-41

Al-SBA-15

-0,2

-0,2

2

4

6

8

10

2

pH

4

6

8

10

pH

Figure 5 The surface charge density vs. pH for (A) the functionalized and pure MCM-41, (B) the functionalized and pure SBA15.

2.5 Surface wettability. To investigate the surface properties of synthesized molecular sieves, droplets of eight different liquids were deposited on aluminosilica and silica tablets. Before that, we have performed the measurements of contact angles of studied liquids deposited on non-porous planar silica surface. In the case of the planar smooth silica material, the glass type KS Kavalier from Megan Poland was used, as it is described elsewhere [41]. The contact angles were measured using Phoenix 300 Contact Angle Analyzer, SEO, whose description was included in our earlier work [41]. Recently [33, 40] on the basis of a corresponding states analysis (R. Radhakrishnan et al. 2002; K.E. Gubbins et al. 2014) it was shown that the molecular level measure of wettability can be described by a microscopic wetting parameter αw. The tendency of an adsorbate to wet a solid substrate may be considered in term of the wetting parameter αw, expressed as [33]:

α w = ρ sσ as 2 ∆

ε as ε aa

(3)


14

Page 15 of 32

where ε is an energy depth and σ is collision length of the Lennard-Jones potential, ρs denotes the number of solid atoms per unit volume and ∆ the distance between separating layers in the substrate. It was shown on the basis of the contact angle measurements [41], that for αw → 0 the system is practically not wetting and for large αw values the system is very well wetting. In Fig. 6 the measured contact angles are shown vs. the microscopic wetting parameter αw for: the smooth planar silica surface (blue curve) and mesoporous materials (red curve). Values of αw for the liquids deposited on an ideal silica surface are known in the literature [33, 40, 42]. An accuracy of contact angles measurements is equal to ± 1 deg. The values of measured contact angles on both mesoporous: silica and Al-silica tablets and also on bulk non-porous silica surface are collected in the table 5.

A

B

190 180 total non-wetting 170 160 150 Ga Hg 140 130 Ga Hg 120 110 100 90 80 PEG 10 000 70 PEG 10 000 60 H2O 50 D2O 40 H2O 30 D2O 20 10 0,0

0,2

0,4

0,6

contact angles on the MCM-41 tablets

Al-MCM-41 tablet bulk silica, non-porous

θbulk=13,69+169,41exp(−(α−0,002) −0,002)/0,19) /0,19) θtablet=8,81+172,41exp(−(α−0,005) −0,005)/0,15) /0,15) θ t, θ bulk [deg]

θ t, θ bulk [deg]

contact angles on the Al-MCM-41 tablets

CCl4 CCl4

0,8

1,0

1,2

C6H14

OMCTS OMCTS

C6H14

1,4

1,6

1,8

2,0

2,2

190 180 total non-wetting 170 160 150 Ga Hg 140 Hg 130 Ga 120 110 100 90 80 PEG 10 000 70 PEG 10 000 60 H2O 50 D2O 40 H2O 30 D2O 20 10

2,4

0,0

0,2

0,4

0,6

MCM-41 tablet bulk silica, non-porous

θbulk=13,69+169,41exp(−(α−0,002) −0,002)/0,19) /0,19) θtablet=9,97+171,69exp(−(α−0,006) −0,006)/0,16) /0,16)

CCl4

C

0,8

1,0

1,2

D

0,0

0,2

0,4

0,6

θbulk=13,69+169,41exp(−(α−0,002) −0,002)/0,19) /0,19) θ t, θ bulk [deg]

θtablet=9,05+172,09exp(−(α−0,005) −0,005)/0,16) /0,16)

CCl4

0,8

1,0

1,2

1,6

1,8

2,0

2,2

2,4

contact angles on the SBA-15 tablets

Al-SBA-15 tablet bulk silica, non-porous

CCl4

1,4

OMCTS OMCTS

αw

contact angles on the Al-SBA-15 tablets

190 180 total non-wetting 170 160 150 Ga Hg 140 Hg 130 Ga 120 110 100 90 80 PEG 10 000 70 PEG 10 000 60 H2O 50 D2O 40 H2O 30 D2O 20 10

C6H14 C6H14

CCl4

αw

θ t, θ bulk [deg]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

C6H14 C6H14

1,4

OMCTS OMCTS

1,6

1,8

2,0

2,2

2,4

190 total non-wetting 180 170 160 150 Ga Hg 140 130 Ga Hg 120 110 100 90 80 PEG 10 000 70 PEG 10 000 60 H2O 50 D2O 40 H2O 30 D2O 20 10 0.0

0.2

0.4

αw

0.6

SBA-15 tablet bulk silica, non-porous

θbulk=13,69+169,41exp(− −(α α−0,002)/0,19 −0,002 /0,19) /0,19 θtablet=9,39+171,90exp(− −(α α−0,005)/0,16 −0,005 /0,16) /0,16

CCl4 CCl4

0.8

1.0

1.2

C6H14

OMCTS OMCTS

C6H14

1.4

1.6

1.8

2.0

2.2

αw

Figure 6 Experimentally measured contact angles for various liquids on the smooth planar silica surface (θbulk, blue points and curve) and on mesoporous surfaces (θt, red points and curve): (A) Al-MCM-41 tablet, (B) MCM-41 tablet, (C) Al-SBA-15 tablet and (D) SBA-15 tablet. The curves are fitted to the points with the exponential functions shown. Insets show the shape of the mercury droplet ((A), (B)) and the n-hexane droplet ((C), (D)) deposited on these tablets.


15

2.4

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 32

Table 5 The values of contact angles on the mesoporous silica and aluminosilica tablets and also on the smooth planar silica surface with the wetting parameters of studied liquids Liquids

bulk silica

θt [°]

αw

bulk silica

MCM-41

SBA-15

Al-MCM-41

Al-SBA-15

OMCTS

2.13

12.73

12.13

11.88

11.08

11.52

H2O

0.28

52.77

39.73

35.68

35.69

33.66

D2O

0.28

45.42

27.47

26.42

23.69

23.77

CCl4

1.18

19.12

12.20

11.36

11.30

11.15

C6H14

1.34

12.60

12.35

12.03

10.86

11.89

PEG 10 000

0.2147*

69.08

59.58

63.72

55.41

61.56

Hg

0.0713*

141.1

139.32

137.42

133.60

135.48

Ga

0.0557*

141.57

136.52

135.72

132.82

133.08

*these values were estimated through an interpolation from θbulk(αw) function

As follows from Fig. 6 and table 5, we have observed the reduction of the contact angle on the rough mesoporous surfaces (tablets surface) for all studied matrices (Al-functionalized and silica substrates) relatively to the smooth planar silica surface. We observe that for non-wetting systems ( small values of αw) the reduction of contact angle is more significant than for well wetting systems (large values of αw). Such a decrease of the contact angle on the rough surface relatively to an ideal substrate can be explained based on the Wenzel model [43-44]. Accordingly to the Wenzel model, the droplet follows the inhomogeneities of the substrate, penetrating them, as it is shown schematically in Figure 7. In this regime, the equation for the measured apparent contact angle θ* on a rough surface can be expressed:

cosθ *= r cos θ

(4)

where θ is a contact angle on an ideal surface and r is the roughness factor (r≥1) defined as the ratio of the actual area of rough surface to the area of equivalent geometric surface. This equation is correct only if the drop size is larger or comparable to the roughness length scale.


16

Page 17 of 32

Figure 7. The homogeneous wetting regime on a rough surface.

In Fig. 8 the results of SEM observations for Al-MCM-41 are presented. The size of macroroughness or irregularities present on the tablets remained at a constant level from several to several tens of microns. The similar SEM image was obtained for all tablets of porous systems studied. It follows from the same procedure of tablets preparation – all samples were compressed in a press under the same pressure ca. 700 MPa.

A

B

25

Average area: 0.97 ± 0.05 µm

2

20

Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

15

10

5

0 0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

2

Area [µm ]

Figure 8 (A) SEM topography of Al-MCM-41 tablet, (B) the histogram of the macropores size distribution.

Therefore, we assume that an observed reduction of contact angles on all studied tablets resulting from the spreading of the droplet on inhomogeneities maintained also at a constant level (range of 1 um) for the topography of all tablets studied and is lower than average drop sizes. In Fig. 9 the dependences of contact angles θt values on all silica tablets versus αw parameter for all liquids studied are presented. As follows from Fig.9, the contact angles for aluminosilica surfaces are smaller than those for the silica surfaces for studied systems. It can result from extra-framework charge-compensating cations or proton species present on the pore wall, known as Lewis or Brønsted acid sites [2,37,45-46], whose increase the interaction


17

Langmuir

between the silica surface and liquid molecule. Because of the amount of aluminium incorporated into the MCM-41 framework is higher than in SBA-15, the acid sites improve the adsorptive properties of the Al-MCM-41 matrix and the reduction of contact angles for the studied liquids on Al-MCM-41 is more significant than on the Al-SBA-15 substrate.

140

Hg Ga

Al-MCM-41 MCM-41

120 100

θt [deg]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 32

80 PEG 10 000

60 40

H 2O CCl4

D 2O

20

0,2

0,4

0,6

0,8

1,0

1,2

C6H14

1,4

OMCTS

1,6

1,8

2,0

2,2

αw

Figure 9 The dependences of contact angles θt versus wetting parameter αw for: porous MCM-41 silica and aluminosilica: Al-MCM-41 tablet.

On the basis of the values of measured contact angles θt on the studied tablets and values of surface tensions γ of the liquids, we have calculated the wetting energy We, defined as [47]:

We = γ cosθ t

(5)

In the Fig. 10, the calculated wetting energies for all studied liquids deposited on the silica and aluminosilica mesoporous surfaces are presented. The values of calculated wetting energies for mesoporous: silica and Al-silica tablets are presented in the Table 6. As follows from Fig. 10A and Table 6, the Al-MCM-41 possess much more wetting energy, and thus it needs higher energy ( per unit area) to break away the substance from its substrate than MCM-41 sample. The Al-SBA-15 surface has nearly the same wetting energy as pure SBA-15 (Fig. 10B). It means


18

Page 19 of 32

that higher acidity better spreads the droplet on the substrate, improving wetting properties, especially for the systems with the small αw values. It confirms the fact, that higher load of aluminium improves significantly both: the wettability and adhesion properties of silica matrix. By Al-functionalization of silica molecular sieves we can create the systems with stronger adsorptive properties.

AA

BB

100 D2O H2O D2O H2O

0

CCl4 CCl4

C6H14

100

OMCTS

D2O H2O

OMCTS

C6H14

D2O H2O

CCl4

0

-100

CCl4

C6H14

OMCTS OMCTS

C6H14

-100 2

We [mJ/m ]

2

We [mJ/m ]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

-200

-300

-200

-300 Hg Hg

Hg

-400

Hg

-400

MCM-41 tablet Al-MCM-41 tablet

-500

-500

Ga Ga

0,0

SBA-15 tablet Al-SBA-15 tablet

Ga Ga

-600

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

2,0

2,2

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

αw

αw

Figure 10 The dependence of wetting energies vs wetting parameter for various liquids on: (A) the functionalized and pure MCM-41, (B) the functionalized and pure SBA-15.

Table 6 The values of wetting energies on the surfaces of mesoporous: silica (MCM-41 and SBA-15) and aluminosilica (Al-MCM-41 and Al-SBA-15) tablets with the wetting parameters of studied liquids Liquids

We [mJ/m2]

Bulk silica αw

MCM-41

SBA-15

Al-MCM-41

Al-SBA-15

OMCTS

2.13

18.08

18.10

18.15

18.13

H2O

0.28

55.83

59.08

59.09

60.55

D2O

0.28

63.78

64.41

65.87

65.83

CCl4

1.18

26.19

26.27

26.28

26.29

C6H14

1.34

18.00

18.02

18.10

18.03

PEG 10 000

0.2147*

-

-

-

-

Hg

0.0713

-368.95

-356.89

-335.53

-346.86

Ga

0.0557

-543.22

-546.04

-520.69

-514.34


19

1,8

2,0

2,2

Langmuir

* no information in the literature about the value of the surface tension

2.6 Melting of water confined in Al-MCM-41 and MCM-41. To investigate an influence of acidity on the confinement effects of water in Al-MCM-41 pores, the DRS and DSC methods were used. To compare the obtained results of melting temperature measurements of water in the Al-MCM-41 with the melting temperature of water placed in pure MCM-41 materials with the same pore size of 14 nm, we have synthesized the silica MCM-41. The obtained material was characterized by nitrogen sorption and TEM microscopy. Large pores MCM material (MCM-41-TMB) was obtained using CTAB surfactant and tetraethoxysilane as a silicon source. 2 g of CTAB was dissolved into the solution of 20 mL deionized water and 2 mL of NaCl (5M), followed by the addition of 4.56 g of tetraethyl orthosilicate and TMB as a swelling agent (1.16 g). The mixture was loaded into an autoclave and heated at 95oC for 72 h. After synthesis the organic parts of support were removed by calcination at 550oC for 4 hours under an air-atmosphere. The results of pore characterization of large pores MCM-41 material are presented in Figure 11. Nitrogen sorption isotherm shown in Fig. 11A, displaying type IV of IUPAC classification isotherms with H2 hysteresis loop. The pore size calculated from the adsorption data (Fig. 11B) indicate a pore diameter of 14 nm.

A

B

Isotherm Linear Plot 600

BJH Adsorption and Desorption dV/dlogD Pore Volume Halsey: Faas Correction

0,0030

MCM-41-TMB (ads)

dV/dlogD Pore Volume [cm /g Α]

MCM-41-TMB (des)

MCM-41-TMB (ads)

C

0,0025

3

500

3

Quantity Adsorbed (cm /g STP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 32

400

300

200

100

0,0020

0,0015

0,0010

0,0005

0 0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

Relative Pressure (p/p0)

0,8

0,9

1,0

1,1

0,0000 0

100

200

300

400

500

600

700

800

Pore Diameter [A]

Figure 11 MCM-41-TMB silica porosity characteristics: (A) Nitrogen sorption isotherms at 77 K. (B) Differential distribution functions of mesopore volumes calculated from nitrogen adsorption isotherms, (C) TEM image of MCM-41-TMB.


20

Page 21 of 32

In the Figure 11C TEM micrograph of MCM-41-TMB is shown. The image confirms an existence of large pores of the sample structure. The results of the dielectric measurements of water placed in Al-MCM-41during the heating process are presented in Fig.12A. As it is shown in Fig. 12A, the capacitance C as a function of T shows a sharp increase at T= 266 K, which corresponds to the melting of adsorbed water inside the pores of Al-MCM-41, and the second increase of C(T) function at T=273 K is related to the melting point of the bulk water. The studied samples are a suspension of filled Al-silica sieves in pure liquid, therefore the observed signals come from both: bulk and confined liquid. This results are consistent with the DSC results, what is depicted in Fig. 12B. The melting temperatures were determined from the positions of the first derivative of the endothermic peaks on the heat flow signals during the heating. The smaller peak at T=265.4 K indicates melting of water inside the Al-MCM-41 and the large peak at T= 273.1 K is related to melting a bulk water. We can observe that the melting temperature of water in Al-MCM-41 pores is depressed compared to the bulk by ∆T=Tm,pore-Tm,bulk= - 7.7 K.

A

B 5 36

Al-MCM-41 + H2O, heating 5 K/min

623 kHz 0

34

273 K

-5

30 28

266 K melting inside the pores

26

Heat Flow [mW]

bulk melting

32

C [pF]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

-10 -15

265.4 K melting inside the pores -20

24

-25

273.1 K

22

-30 200

210

220

230

240

250

260

270

280

290

300

bulk melting

exo

170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320

T [K]

T [K]

Figure 12 (A) The temperature dependence of capacitance C and (B) the DSC scan: for water confined in Al-MCM-41 mesopores of 14 nm diameter.

On the basis of C(T) measurements obtained in the whole range of the applied frequencies f , the values of ε′ and ε″ were calculated; the spectrum plots of ε′, ε″ vs. log f are presented in Figs. 13A and 13B. The frequency spectrum at a given temperature, by fitting to the Debye dispersion relation (Eq. 2)., allow us to obtain the values of


21

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 32

the orientational relaxation time τ before and after phase transition temperatures in the pores. The relaxation time was obtained as the reciprocal of the frequency related with a saddle point of the ε′ function or a maximum of the ε″ function [36]. The spectrum presented in Fig. 13A at T=231 K shows two relaxation mechanisms in the crystal phase of water with the two relaxation times of the order of 10-3 s and 10-6 s. The relaxation time about τ ~ 10-3 s can describe the relaxation process of the hexagonal ice typical for the bulk ice [48] and the shorter component can describes the relaxation time of cubic ice confined into the mesopores of

Al-MCM-41, as it was shown in the

previous paper [48-49]. The corresponding Cole-Cole diagram of ε″ vs. ε′ at T=231 K is shown in Fig.13C. As it is shown in this diagram, the first, smaller semicircle, corresponds to response of confined ice, while the second, characterizes the relaxation of the hexagonal ice [48]. Above the melting point inside the pores, at T=270K, the frequency spectrum of ε′ and ε″ in Fig. 13B, shows another two relaxation mechanisms described also by a Cole– Cole distribution of relaxation times (Fig.13D). The component of the order of 10-6 s describes Ih bulk ice, while the second component, of order 10-4 to 10-2 s, temperature dependent, characterizes the conductivity of water placed in the pores [48]. The relaxation time about 10-3 s visible at the temperatures above the bulk melting point is related with an interfacial Maxwell-Wagner polarization occurring in the case of inhomogeneous systems. Figure 13E presents the temperature dependence of the dielectric relaxation time of water confined in the Al-MCM-41 mesopores based on fitting formula (2) to the dispersion spectrum. For temperatures below 265 K, two components of the relaxation time τ are visible: one branch of them is strongly varying with the temperature from 10-2 to 10-6 s, and is typical for the hexagonal ice Ih in the bulk phase [48]. The second branch - 10-6 s, appears immediately below the pore melting point and can be related to the cubic Ic form of ice confined in the pores [48]. As follows from the recent work of Domin et al. [49-50], the metastable ice formed in pores is a combination of cubic sequences intertwined with hexagonal sequences stabilized by the confinement. The theory [51] predicts that when a water droplet freezes, a two-dimensional nucleation process occurs. As a result, the layers of hexagonal and cubic ice structures grow alternately in the crystal (in various combinations), so the resulting ice is made up of a combination of intertwined cubic and hexagonal stacking sequences; the content of cubic form of ice in the pores depends of the pore size. This results suggest that at lower temperatures, below melting point inside the pores, the stable cubic ice forms Ic exists inside the Al-MCM-41 pores.


22

Page 23 of 32

A

T=231 K

B

90

0,18

80

0,16 70

ε`

T=270 K

0,20

ε`

0,14

60

0,12

50

0,10

80

0,20

78

0,18

76

0,16 0,14

ε``

74

0,12

72

ε``

0,10

ε`

0,08

40

70 0,08

0,06

68

0,04

66

20

0,02

64

0,02

10

0,00

62

0,00

30

ε``

-1,0

-0,5

0,0

0,5

1,0

1,5

2,0

2,5

3,0

0,04

-0,02 -1,0

-0,5

0,0

0,5

1,0

1,5

2,0

2,5

logf [kHz]

C

T=231 K

D

T=270 K

267.2 K melting inside the pores

E 0,01

relaxation of bulk ice

1E-3

Maxwell-Wagner relaxation

1E-4

1E-5 relaxation of confined ice

bulk melting

1E-6

1E-7 190

200

210

0,06

ε`

60

logf [kHz]

logτ [s]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

220 230 240 250 260 270 ACS Paragon Plus Environment

280

290

300

T [K]

23

3,0

ε``

Langmuir

Figure 13 The spectrum plot of ε` and ε`` vs frequency f for the water confined inside the Al-MCM-41 mesopores of 14 nm diameter at: (A) 231 K, (B) 270 K and the Cole-Cole representation diagrams at: (C) 231 K, (D) 270 K, (E) dielectric relaxation time τ versus temperature for the water confined inside the Al-MCM-41 system.

By using DSC method, the melting point of water confined inside the MCM-41-TMB with 14 nm pore size was estimated from the positions of the first derivative of the endothermic peaks on the heat flow signals during the heating. The presented in Fig. 14 DSC scan has showed the smaller peak at T=267 K related to the melting of water inside the pores of MCM-41-TMB and the large peak at T= 272.4 K which is corresponding to the bulk melting point.

5 MCM-41-TMB + H2O, heating 5K/min

0

Heat Flow [mW]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 32

-5

267 K melting inside the pores -10

-15

-20

272.4 K bulk melting -25

exo 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320

T [K] Figure 14 The DSC scan for water confined in MCM-41-TMB mesopores of 14 nm diameter.

As follows from Fig. 14, the melting point depression ∆Tp of water in MCM-41-TMB silica pores of 14 nm of diameter is equal - 5.4 K. For water in Al-MCM-41 the melting point depression is - 7.7 K. It suggests that an introduction of aluminium into the framework of silica leads to the depression of the melting temperature of water about 2.3 K, relative to the melting point depression in the pure silica pores studied [50]. The functionalization causes an increase of the value of melting point depression, similarly as the reducing of the pore size effect.


24

Page 25 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

These results are related with the change of the structure of the pores surface, as it was shown by XPS and potentiometric studies, where the existence of Al2O3 and Al(OH)3 groups on the functionalized Al-MCM-41 silica surface was found. In the case of water filled Al-pores it leads to increase of wetting energy (reduction of the contact angle on the surface) relatively to the wetting energy of pure MCM-41 surface and in a consequence to the change of αw parameter of water placed in Al-MCM-41 compare to the αw for water in pure silica pores. As follows from the literature, the melting point temperature in pores can be considered in terms of two parameters: the pore size H and wettability parameter αw. If the pore width H is the same, the change of αw value in the system can lead to the change of the melting temperature in pores. The results obtained here confirm this expectation.

CONCLUSIONS We report the synthesis and characterization of aluminosilica and silica porous matrices: MCM-41. It is known that the functionalization of silica by metals such as: aluminium, titanium, vanadium or chromium [19-20] is widely investigated in order to achieve the best adsorptive and catalytic properties, e.g. high adsorption capacity or to design catalysts for conversion reactions [52]. The aim of our work was to study an influence of acidity on the surface properties of mesoporous molecular sieves, their wettability and the melting behavior of water confined in the pores. For this purpose, MCM-41 and Al-containing MCM-41 silica with a molar ratio of (n(Si)/n(Al)=50) and with high-ordered hexagonal mesoporous structure, were acquired by a sol-gel method using CTAB as pore creating agent and characterized. Aluminium was introduced into the framework of a silicate material by direct synthesis method. The EDS results have confirmed the assumed in synthesis gel content of aluminium and XPS investigations have provided an information that aluminium is involved in creation Al(OH)3 and Al2O3 functional groups on the surface. Potentiometric titration measurements conducted for Al-SBA-15 and for Al-MCM-41 have shown the significant pHpzc shift towards lower values of pH for Al-MCM-41, while for Al-SBA-15 with significantly lower content of Al, this shift is not observed. The wetting properties of synthesized samples were also analyzed. The measured contact angles for several liquids on silica and on Al-silica tablets have indicated the pronounced improvement of wettability for samples of functionalized MCM-41..


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We have also investigated the melting behaviour of water confined in Al-MCM-41 and MCM-41 by using dielectric relaxation spectroscopy (DRS) and DSC methods. The results revealed that the melting point of water in the Al-functionalized pores is depressed about 2.3 K compared to the melting point in pure silica pores with the same pore size of 14 nm. This effect results from the change of pore-walls interactions of aluminosilica matrix. The presented results have indicated an improvement of adhesion effects on the surface of aluminium functionalized silica porous systems. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENTS The authors gratefully acknowledge Dr. Magdalena Błachnio for help with potentiometric titration measurements. We are also grateful to Dr. Zbigniew Fojud and Dr. Marcin Jarek for their help with DSC measurements. We would like to thank for financial support for National Center of Science grant No. DEC-2013/09/B/ST4/03711; we are also grateful for partial support from the Poland Operational Program ‘Human Capital’ PO KL 4.1.1 and the National Centre for Research and Development under research grant No. PBS1/A9/13/2012.

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Surface Properties of Al-Functionalized Mesoporous MCM-41 and the

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